The present invention relates generally to interface devices between humans and computers, and more particularly to computer interface devices that provide force feedback to the user.
Interface devices are used extensively with computer systems in the implementation of computer-controlled games, simulations, and other applications very popular with the mass market of home consumers. In a typical implementation, a computer system displays a visual environment to a user on a display device. Users can interact with the displayed environment by inputting commands or data from the interface device. Popular interface devices include joysticks, xe2x80x9cjoypadxe2x80x9d button controllers, mice, trackballs, styluses, tablets, pressure spheres, foot or hand pedals, or the like, that are connected to the computer system controlling the displayed environment. The computer updates the environment in response to the user""s manipulation of a moved manipulandum such as a joystick handle or mouse, and provides visual feedback to the user using the display screen.
In some interface devices, haptic (e.g., tactile) feedback is also provided to the user, more generally known as xe2x80x9cforce feedback.xe2x80x9d These types of interface devices can provide physical sensations to the user manipulating the physical object of the interface device. Typically, motors or other actuators of the interface device are coupled to the manipulandum and are connected to the controlling computer system. The computer system receives sensor signals from the interface device and sends appropriate force feedback control signals to the actuators in conjunction with host events. The actuators then provide forces on the manipulandum. A local microprocessor can be used to offload some computational burden on the host. The computer system can thus convey physical sensations to the user in conjunction with other visual and auditory feedback as the user is contacting the manipulandum. Commercially available force feedback devices include the ForceFX joystick from CH Products, Inc. and Immersion Corporation, and the Sidewinder Force Feedback Pro from Microsoft Corporation.
One problem occurring in providing commercially available force feedback devices with realistic forces is providing a low cost device. Such components as belt drive transmissions can be used to reduce manufacturing costs. However, one problem occurring with many types of belt drives is that an amount of compliance or backlash is typically inherent in the system caused by the flexibility or stretching of the belts. Other types of transmissions also may introduce compliance into a system, as well as various types of linkages or gimbal mechanisms which provide the degrees of freedom to the manipulandum of the force feedback device. The compliance can also be derived from plastic or other flexible components used in low-cost devices.
The compliance and backlash in a force feedback mechanical system can cause problems in accurately sensing the position of the manipulandum. This can be a particular problem in those systems having significant compliance between the manipulandum and the sensor. The user may have moved the manipulandum a small distance, but due to the compliance this change, in position is only partially detected or not detected at all by the sensor, or is detected too long after the event for the device to provide meaningful forces in reaction to the change in position. This is especially of concern when the position sensor is rigidly coupled to the actuator to sense motion by sensing rotation or movement of the actuator shaft (and where the manipulandum is compliant-coupled to the sensor), as is commonly done in force feedback devices to provide greater sensing resolution with a given sensor and to provide more stable control of the device.
Another problem involved with inaccurate position reporting in a force feedback device is related to sensing the position of the manipulandum near the limits to provided degrees of freedom. For example, force feedback devices typically provide hard stops to limit the motion of the manipulandum to a constrained range. Due to compliance in the mechanical and/or drive system, the problem of sensing the position of the manipulandum is exacerbated at the hard stops. For example, when the user moves the manipulandum fast against the hard stop, the compliance in the system may allow further motion past the hard stop to be sensed by the sensor due to compliance and inertia. However, when the manipulandum is moved slowly, the inertia is not as strong, and the sensor may not read as much extra motion past the hard stop. These two situations can cause problems in sensing an accurate position consistently.
Yet another problem with position sensing can occur upon startup of a force feedback device. If a device uses relative or incremental sensors, as many force feedback devices do, then a controlling microprocessor or host computer does not immediately know the starting position of the manipulandum when the device is first powered. This can cause problems when defining a range of motion for the manipulandum. The assumption that the manipulandum is at the center of the full range of motion can cause problems since the startup position may actually be very close to or at a limit such as a hard stop, and the manipulandum cannot be moved very far before this limit is reached even though the controller expects a much larger range of motion. Dynamic calibration can be used, where the range of the device is considered nominal at startup and is gradually increased as the sensors detect the manipulandum at ever-increasing ranges. However, a problem can exist for force feedback devices that provide this type of dynamic calibration and which use a software centering spring upon startup, which is not a physical spring but a spring if force controlled by the device and output by the actuators which centers the manipulandum in its range of motion. If the range of the manipulandum is made small and then allowed to increase, then the default spring at startup will cause instability in the device, i.e., the manipulandum will oscillate due to the device sensing tiny motions as large motions within the small range, which causes the effective gain of the control loop to be too high for the position range.
The present invention provides improvements in the sensing of position of a manipulandum of a force feedback device. The features of the present invention are useful for more accurately sensing manipulandum position of a force feedback device that includes compliance in its mechanical systems, and for calibrating a force feedback device having relative sensors.
More particularly, one aspect of the present invention compensates for sensing inaccuracies contributed to by compliance in the mechanical systems of a force feedback device is provided. The force feedback device is coupled to a host computer and includes at least one actuator for outputting forces and a sensor. A raw sensor value of a position of a manipulandum of the force feedback device is read in a range of motion of the manipulandum, the manipulandum, such as a joystick handle, being grasped by a user. The raw sensor value is adjusted based on a compliance of the force feedback device between sensor and manipulandum, where the adjustment compensates for the compliance to provide a more accurate position of the manipulandum. The adjusted sensor value is used as the position of the manipulandum when, for example, updating an application program implemented by the host computer. Preferably, a microprocessor local to the force feedback device adjusts the sensor value and reports the adjusted sensor value to the host computer.
The adjusting of the raw sensor value preferably includes adjusting the raw sensor value based on a compliance constant and a current output force, where the compliance constant has been previously determined. When the force feedback device performs the adjustment, the adjusted sensor value is reported to the host computer as the position of the manipulandum. The raw (unadjusted) sensor value can be used to determine closed-loop position-dependent forces by, for example, a microprocessor local to the interface device. The sensor can be coupled to the actuator such that the sensor detects movement of an actuator shaft. The force feedback device can include a variety of linkages and transmission systems, such as a belt drive for transmitting forces from the actuator to the manipulandum.
In another aspect of the present invention, a range of motion of a manipulandum of a force feedback device is dynamically calibrated, where the force feedback device is coupled to a host computer and includes an actuator and at least one relative sensor. A predetermined initial range of movement for the manipulandum is assigned when the force feedback device is initially powered. The initial range includes two boundary values, a maximum value and a minimum value. A sensor value representing a position of the manipulandum in the range of movement is received as the manipulandum is moved. The maximum value or minimum value is set to the received sensor value if the received sensor value is outside the initial range. The other boundary value not set to the received sensor value is adjusted to maintain the initial range between the maximum value and the minimum value unless this other boundary value has been previously detected outside the initial range. This allows the initial range to be maintained until new maximum and minimum points are detected dynamically. The initial range is greater than zero and is less than an entire physical range of motion of the manipulandum to confer stability on the device upon startup, where the manipulandum is considered to be positioned at about a center of the initial range when the force feedback device is initially powered.
Another aspect of the present invention provides accurate sensing of position of a manipulandum in a force feedback device that includes compliance between the manipulandum and a position sensor of the force feedback device using filtering. A raw sensor value of a position of a manipulandum is read in a range of motion of the manipulandum that is grasped by a user. The raw sensor value is filtered for overshoot sensor values occurring at limits to the range of motion of said manipulandum. The range of motion of the manipulandum is dynamically calibrated by adjusting minimum and maximum values of the range of motion based on the extent of motion of the manipulandum and using the filtered sensor value. The filtering can, for example, use a low pass filter on the raw sensor data. Preferably, the unfiltered raw sensor value is used for determining a position of the manipulandum in the range of motion. The dynamic calibration also may include assigning an initial range with initial maximum and initial minimum values and maintaining the initial range between the minimum and maximum values until both minimum and maximum values are detected outside the initial range.
In another aspect of the present invention, sensing inaccuracies contributed to by compliance in the mechanical systems of a force feedback device are compensated for by using a normalization procedure. A raw sensor value is read describing a position of a manipulandum in a range of motion. The raw sensor value is normalized to a normalized range of motion, including providing a saturation zone at each end of the normalized range that adjusts sensor values over a saturation level to the saturation level, where the saturation levels are provided at the ends of the normalized range. The normalized sensor value is reported to the host computer, and the host computer updates an application program using the normalized sensor value. The normalizing can use a normalizing function, such as a linear function having saturation levels at its ends. If the raw sensor value is adjusted based on a compliance of the force feedback device as above, the adjusted sensor value is preferably normalized to the normalized range of motion and is reported to the host computer. The raw, unadjusted sensor value can be normalized and used for local closed-loop-determination of forces by a local microprocessor.
The improvements of the present invention provide more accurate sensing of the position of the manipulandum in a force feedback device, and are especially applicable to low cost force feedback devices provided for competitive consumer markets, in which compliance in the mechanical system can be significant. The compliance compensation, filtering, and normalization features of the present invention provide accurate positions of the manipulandum to the host computer regardless of compliance between manipulandum and sensor, and regardless of other characteristics in the sensors, actuators, and mechanical system leading to inaccurate position sensing. The dynamic calibration of the present invention provides accurate calibration for relative sensors and prevents instability of the device at startup. These improvements allow a low-cost force feedback device to provide more precise position sensing and more realistic force sensations to the user.